This invention pertains generally to the field of surgical instruments and similar devices, to micromechanical systems, and to ultrasonically actuated instruments.
Various types of hypodermic needles have been developed for the injection of substances, e.g., drugs, into the body, or the withdrawal of body fluid samples, e.g., blood. For drawing blood samples from a vein, the subject""s blood pressure may be sufficient to pump the blood sample through the bore of the hypodermic needle to a container. In situations where a needle is used to withdraw samples of other body fluids, it is generally necessary to use some type of mechanism to draw or pump the body fluid through the needle to a container, for example, a plunger attached to the needle which can be manipulated to provide a sufficient vacuum draw on the needle to draw the fluid sample through the bore of the needle into the container of the plunger.
While conventional hypodermic needles that can be manipulated by hand by medical personnel have been successfully used for many years for withdrawing relatively large samples of blood and other body fluids, the use of such needles is, of course, accompanied by some tissue damage as the needle punctures the skin and subdermal tissues of the subject. This incidental tissue damage becomes particularly a problem if repeated sampling is required, for example, for monitoring of blood sugars by diabetics or in other situations where it is desirable to be able to closely monitor the levels of blood components or drug levels within a patient. Efforts have thus been directed to the development of miniaturized needle systems which are capable of very lightly puncturing the skin, and in some cases limiting penetration to the dermal layer of skin to collect interstitial fluid in the dermal layer without puncturing capillaries beneath the dermal layer. Examples of such systems are shown in, e.g., U.S. Pat. Nos. 5,582,184, 5,682,233, 5,746,217 and 5,820,570. The small size of the needles and the small amounts of interstitial fluids that are withdrawn through the needle complicate the problem of drawing fluids through the needle to a sample container or a sample chamber.
It has been found that ultrasonically actuated needles will pump fluid from the distal end of the needle to its proximal end under appropriate conditions. Such needles can also be fabricated using microelectromechnical fabrication techniques. See, A. Lal, et al., xe2x80x9cSilicon Micro-Fabricated Horns for Power Ultrasonics,xe2x80x9d Transducers 95, June 1995, Stockholm, Sweden; A. Lal, et al., xe2x80x9cMicromachined Silicon Ultrasonic Needle,xe2x80x9d IEEE Ultrasonics Symposium, Nov. 7-10, 1995, Seattle, Wash., U.S.A.; and U.S. Pat. Nos. 5,569,968 and 5,728,089 to Lal, et al. A limitation of the use of such ultrasonically actuated needles for body fluid sampling is the potential damping of the vibration of the needle as it is inserted into relatively stiff skin tissue. The damping of the vibrations of the needle by skin tissue reduces the efficiency of pumping of fluid from the needle. Further, increasing the amplitude of the vibrations applied to the needle to overcome the loss of efficiency may result in excessive disruption of the body tissue penetrated by the needle by virtue of the mechanical displacement, cavitation, and localized heating of tissue caused by the vibrations.
In accordance with the present invention, an ultrasonically actuated needle pump system provides efficient pumping of body fluids with minimal disruption of body tissue adjacent to the needle. Ultrasonic pumping action occurs through the needle system without interference with the pumping action by the skin or other body tissue which is penetrated. The needle system is well-suited to the extraction of precisely controlled amounts of fluid for purposes such as sampling of interstitial fluid.
The needle pump system of the invention includes an outer, stationary tubular needle having a distal end, which may have a penetrating tip, and a hollow bore. An inner tubular needle is mounted in the bore of the outer needle. The inner and outer needles may be formed as any type of elongated hollow structure acting as a conduit for fluid, and may have any desired cross-sectional shape in addition to the common round inner and outer surfaces for such conduits. The distal end of the inner tubular needle is located adjacent to, at or spaced slightly away from, the distal end of the outer needle. An ultrasonic actuator is coupled to the inner tubular needle at a position away from the distal end to selectively drive the inner needle in longitudinal mode vibrations at ultrasonic frequencies. The inner needle is free to vibrate independently of the outer needle, and the outer needle is not vibrated and is essentially stationary. The outer needle may, for example, be formed of metal in a conventional fashion with a sharp, penetrating tip, and the inner tubular needle may be formed of a capillary tube which may be formed of a transparent material, such as glass, which facilitates inspection of the contents of the inner needle as it is pumped therethrough. It is found that, in accordance with the invention, the vibration of the inner tubular needle results in highly efficient pumping of fluid from the distal end of the needle system, e.g., at the penetrating tip of the outer needle, to the proximal end opening of the inner tubular needle. Such pumping action occurs when the distal end of the needle system is inserted into a sample of liquid by itself as well as when the needle system is used to penetrate skin tissue. Because the outer needle is stationary, it does not mechanically disrupt or heat the adjacent body tissue, and the surrounding outer needle shields the vibrating inner needle from contact with body tissue so that no damping of the vibrations of the inner needle occurs.
It is also found in accordance with the invention that the needle system of the invention may be utilized to pump fluid from a container at the distal end and to efficiently atomize the liquid at an open proximal end of the inner needle.